Pickup tube for television and the like



1952 D. WEIGHTON PICKUP TUBE FOR TELEVISION AND THE LIKE Filed Sent. 14, 1949 Iqvenlor DONALD WE/QH rou as By +45% Attorney;

Patented Nov. 25, 1952 Donald .Weighton, Cambridge, England. assignor to Pye Limited, Cambridge, England, a British company Application September 14, 1949, Serial No. 115,670

In Great Britain September 17, 1948 4 Claims.

The present invention relates to pick-up tube for television and the like of the image iconoscope type in which the scene to be televised is focussed on to a photo cathode, the electrons omitted from which are guided by the magnetic field of the surrounding coil on to a corresponding point of a target or mosaic electrode. In such tubes as hitherto constructed, the target has been scanned by a beam from an electron gun disposed in a tubular extension at the side of the pick-up tube. With such tubes as hitherto constructed the magnetic lens for directing the electrons omitted from the photo cathode on to the corresponding points of the target introduces geometrical distortions in the picture appearing on the mosaic. Geometrical distortions is also produced by the magnetic field created by the deflecting electrodes for the scanning gun which are disposed around the tubular extension in which the gun is located.

In the operation of image-iconoscopes, both the photo-electrons and the scanning electrons must impinge upon the target with such high velocities, generally of the order of 1000 volts, that more secondary electrons are emitted from the target than the number of arrivin primary electrons. The target stabilises at a potential so that only one secondary electron is collected by the collector electrode for each arriving primary electron, the remaining secondary electrons emitted being redistributed over the surface of the target. The tube thus operates in both the image and scanning sections between the first and second crossover points of the wellknown curve, in which the secondary emission ratio is plotted against the velocity of the primary electrons.

The present invention consists in a pick-up tube of the type referred to in which the geometrical distortions due to the magnetic field of the focus coil and the deflecting coils are eliminated. To this end, the invention from one aspect consists in a pick-up tube comprising an evacuated envelope containing a photo cathode and target onto which electrons emitted from the photocathode are to be directed, the electron gun for providing a scanning beam within the envelope being omitted. The beam for scanning the target toproduce the picture signals in the output circuits connected to the signal plate associated with the target and correspondingto the picture being televised, is generated by causing a flying spot of light to scan the photo cathode of the tube. In this way the scanning beam generated on the photo cathode by the flying spot of light is directed on to the target by the same focus coil as directs the electrons released from the photo cathode by the scene being televised, whereby any distortions produced .by the focus coil alfects not only the image electrons but also the scanning beam inexactly the same way, the distortions thereby being compensated automatically and exactly. The arrangement according to the invention also enables the manufacture of pick-up tubes to be simplified since the tubular extension and the scanning gun are eliminated from the construction of the pick-up tube proper.

Advantageously the scanning light beam is generated by a separate cathode ray tube, an image of the scanning spot on the fluorescent screen being focussed on to the photo cathode of the pick-up tube at the same time as'the optical image to be transmitted is projected thereon by the lens system .of the television camera. I

In order that this invention may be more fully understood, one embodiment thereof will now be described with reference to the accompanying drawings which show diagrammatically one arrangement of a television camera according to the invention.

Referring to the drawing, the television camera comprises a pick-up tube having an evacuated envelope 7 provided at one end with a photo cathode 6 carried by the inner surface of the envelope. Mounted towards the other end of the envelope is a target or mosaic electrode 8 mounted on One side of a support, on the other side of which is a signal plate 9. An accelerating and collecting electrode II is constituted by a conductive coating on the inner surface of the envelope 7 and this accelerating electrode and a surrounding focussing coil [0 are employed to transfer an electron image emitted from the photo cathode 6 to, the target 8. Connections extend outside the envelope from the photo cathode 6, signal plate 9 and accelerating electrode l I.

The electron beam of a cathode ray tube I is focussed by a focus coil 3 and deflected by defiection coils 2 in such a manner that a raster is formed on the fluorescent screen on the upper face of the tube. The light from this screen is reflected by a half silvered mirror or equivalent semi-reflecting surface and focussed by a lens 4 on to the photo cathode 6. At the same time the light from the scene to be transmitted is focussed by a lens IE to form an image on the said photo cathode 6 after passing through the half silvered mirror 5. The apparatus is so adjusted that the moving light beam projected from the cathode ray tube on to the photocathode 6 scans the same area thereof as that occupied by the image focussed thereon of the subject to be televised.

The electrons emitted from the photo cathode 6 owing to the light incident from the scene to be transmitted are accelerated by a sufiiciently high potential difference applied between the photo cathode and the accelerating electrode l I that the photo-electrons impinge on the target with such velocity as to produce a secondary emission ratio which is greater than unity. The electrons from the photo cathode 6 are focussed by the magnetic lens to form an image on the target 8 which ismade of insulating material, preferably mica, so that a charge image is built up on the surface of the target 8 corresponding to the scene which is to be transmitted. At the same time a scanning beam of electrons is generated at the photo cathode 6 due to the light arriving from the cathode ray tube l and this beam of electrons is accelerated and focussed on the surface of the target by the same accelerating electrode H and the same focussing coil It. The beam of scanning electrons thus impinges upon the target with the same velocity as the image photo-electrons. The charges of each small element of surface of the target are discharged in turn by the scanning beam and the discharge current, which constitutes the picture signal, flows through a resistor l2 connected to a signal electrode 9 capacitatively associated with the target 3. Potentials corresponding to the picture signals are developed 2 across the resistor I2 and are fed to an amplifier l4 through a condenser [3.

It will be noted that from the photo cathode E the same means are used for focussing the image to be transmitted and also the scanning beam, thus any distortions which may occur as a result of the electrostatic and electro-magnetic means employed are equal for both and in consequence do not affect the picture signal.

In the arrangement shown in the drawing, a half silvered mirror is used, the optical image of the scene to be transmitted being projected normally on to the photo cathode through this half silvered mirror, while the scanning beam of light is reflected by said mirror 5. As an alternative to this arrangement, an optical image of the raster formed by the moving fluorescent spot on the screen of the cathode ray tube I can be projected obliquely on to the photo cathode 6. The raster on the cathode ray tube screen may be arranged to be of an appropriate shape to provide compensation for the oblique projection on to the cathode 6. Such compensation means are well known.

Whilst one embodiment of this invention has been described with reference to the accompany ing drawings, it has to be understood that if desired the principles of the present invention may be applied to other constructions. For example the target may be formed as a mosaic of discrete particles capable of retaining the charge image. Also if desired the picture signal may be obtained from the accelerating electrode instead of from the signal plate 9. The pick-up tube may also include further accelerating electrodes or auxiliary electrodes such as electron multipliers.

I claim:

1. Television apparatus comprising an evacuated envelope containing a semi-transparent photo-cathode and a secondary emissive target spaced from said photo-cathode, optical means for focussing upon said photo-cathode an optical image of a subject to be televised to cause said photo-cathode to release photo-electrons responsive to the light distribution in said optical image, means for focussing the released photo-electrons as an electron image upon said target, means for accelerating said released photo-electrons towards said target at a high velocity to cause the emission from said target of secondary electrons greater in number than the incident photo-electrons and to cause said target to develop an electrostatic charge image corresponding to said optical image, a light source outside said envelope, and means for scanning the same area of said photo-cathode as that on which said optical image is focussed with a moving spot of light from said light source, the photo-electrons released from said photo-cathode in response to said scanning light spot being focussed and accelerated by said focussing and accelerating means as a scanning electron beam upon said target.

2. Television apparatus comprising an evacuated envelope containing a semi-transparent photo-cathode and a secondary-emissive target spaced from said photo-cathode, optical means for focussing upon said photo-cathode an optical image of a subject to be televised to cause said photo-cathode to release photo-electrons responsive to the light distribution in said optical image, means for focussing the released photo-electrons as anelectron image upon said target, means for accelerating said released photo-electrons towards said target at a high velocity to cause the emission from said target of secondary electrons greater in number than the incident photo-electrons and to cause said target to develop an electrostatic charge image corresponding to said optical image, a cathode'ray tube separate from said envelope having a fluorescent screen, means for causing the electron beam of said cathode ray tube to scan said screen to form a light raster on said screen, and means for focussing said raster upon the same area of said photo-cathode as that upon which said optical image is focussed, the photo-electrons released from said photo-cathode in response to light from said raster being focussed and accelerated by said focussing after accelerating means as a scanning electron beam upon said target.

3. Television apparatus comprising an evacuated envelope containing a semi-transparent photo-cathode and a secondary-emissive target spaced from said photo-cathode, optical means for focussing upon said photo-cathode an optical image of a subject to be televised to cause said photo-cathode to release photo-electrons responsive to the light distribution in said optical image, means for focussing the released photo-electrons asan electron image upon said target, means for accelerating said released photo-electrons towards said target at a high velocity to cause the emission from said target of secondary electrons greater in number than the incident photo-electrons and to cause said target to develop an electrostatic charge image corresponding to said optical image, a translucent reflector between said optical means and said photo-cathode adapted to transmit said optical image, a cathode ray tube having a fluorescentv screen, means for causing the electron beam of said cathode ray tube to scan said screen with an electron beam to form a light raster on said screen, and means for focussing said raster upon said photo-cathode by reflection at said reflector, the photo-electrons released from said photo-cathode in response to 5 light from said raster being focussed and accelerated by said focussing and accelerating means as a scanning electron beam upon said target.

4. Television apparatus comprising an evacuated tube containing a semi-transparent photocathode and a secondary-emissive target spaced from said photo-cathode, optical means for projecting upon said photo-cathode normally thereto an optical image of a subject to be televised to cause said photo-cathode to release photo-electrons responsive to the light distribution in said optical image, means for focussing the released photo-electrons as an electron image upon said target, means for accelerating said released photo-electrons towards said target at a high velocity to cause the emission from said target of secondary electrons greater in number than the incident photo-electrons and to cause said target to develop an electrostatic charge image corresponding to said optical image, a semi-transparent mirror disposed between said optical means and said target to allow transmission of light from said optical image therethrough, means for developing a moving light spot for scanning, means for projecting said light spot upon said mirror, said mirror being arranged to reflect the 9 light from said spot on to said photo-cathode normally thereto for scanning said photo-cathode, the photo-electrons released therefrom in response to said light spot being focussed and accelerated by said focussing and accelerating means as a scanning electron beam upon said target.

DONALD WEIGHI'ON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

